Hi there.
The “Lead ID” variable from the FB Lead Ads module simply won’t work as a Subscriber ID in the Manychat module. I can’t see what is the specific module that you are using, but in general, each app has its own record IDs.
If I understood your description correctly, I would suggest creating the scenario in a way that you first use the email that comes from FB to find out whether the subscriber already exists in Manychat. If yes, update the subscriber or do nothing. And if no, then create a new subscriber.
Cheers,